fix: properly check for null buffer loaded state on review open

[GuillaumeLagrange]

Describe what this PR does / why we need it

Fixes a lua error that happened this way

1. Open review
2. Close review
3. Delete null buffer (for example using <cmd>%bd!|e#|bd!#<cr>)
4. Open review
5. See this error

 Error executing vim.schedule lua callback: ...neovim-plugins/octo.nvim/lua/octo/reviews/file-entry.lua:492: Invalid buffer id: 6
 stack traceback:
         [C]: in function 'nvim_win_set_buf'
         ...neovim-plugins/octo.nvim/lua/octo/reviews/file-entry.lua:492: in function 'load_null_buffer'
         ...neovim-plugins/octo.nvim/lua/octo/reviews/file-entry.lua:485: in function 'load_null_buffers'
         ...ume/neovim-plugins/octo.nvim/lua/octo/reviews/layout.lua:247: in function 'file_safeguard'
         ...ume/neovim-plugins/octo.nvim/lua/octo/reviews/layout.lua:57: in function 'open'
         ...laume/neovim-plugins/octo.nvim/lua/octo/reviews/init.lua:190: in function 'initiate'
         ...laume/neovim-plugins/octo.nvim/lua/octo/reviews/init.lua:123: in function 'callback'
         ...laume/neovim-plugins/octo.nvim/lua/octo/reviews/init.lua:75: in function 'cb'
         .../guillaume/neovim-plugins/octo.nvim/lua/octo/gh/init.lua:164: in function ''
         vim/_editor.lua: in function <vim/_editor.lua:0>

Does this pull request fix one issue?

Describe how you did it

Describe how to verify it

Special notes for reviews

Checklist

• Passing tests and linting standards
• Documentation updates in README.md and doc/octo.txt